#include<iostream>
#include<cstring>
using namespace std;
const int N = 110;
char star[N], aim[N];
int n;
int cnt;
void turn(int k)
{
	if (star[k] == 'o') {
		star[k] = '*';
	}
	else {
		star[k] = 'o';
	}
}
int main()
{
	cin >> star >> aim;
	n = strlen(star);
	for (int i = 0; i < n; i++) {
		if (star[i] != aim[i]) {
			turn(i);
			turn(i + 1);
			cnt++;
		}
	}
	cout << cnt << endl;
	return 0;
}